The present invention relates to a method for producing ultra fine metallic alloy powders, which are environmentally stable. More particularly, it relates to a method for producing substantially nanometer-sized, environmentally stable reactive alloy particles at a high production rate.
The interest in ultra-fine particles, particularly nanometer-sized clusters (dxe2x89xa6200 nm), is due to the unique processing characteristics as well as performance properties exhibited by very small particles of metals, semiconductors and ceramics. Ultra-fine particles have enormous potential in metal and ceramic processing. For example, smaller particles can be sintered at much lower temperatures. Not only the structure, but also the mechanical, electronic, optical, magnetic and thermal properties of nano-crystalline materials are different from those exhibited by their bulk counterparts. Nano-phase metals and ceramics derived from nanometer-scaled particles are known to exhibit unique physical and mechanical properties. The novel properties of nano-crystalline materials are the result of their small residual pore sizes (small intrinsic defect sizes), limited grain size, phase or domain dimensions, and large fraction of atoms residing in interfaces. Specifically, ceramics fabricated from ultra-fine particles are known to possess high strength and toughness because of the ultra-small intrinsic defect sizes and the ability for grain boundaries to undergo a large plastic deformation. In a multi-phase material, limited phase dimensions could imply a limited crack propagation path if the brittle phase is surrounded by ductile phases so the cracks in a brittle phase would not easily reach a critical crack size. In addition, dislocation movement distances in a metal could be limited in ultra fine metallic domains, leading to unusually high strength and hardness. Even with only one constituent phase, nano-crystalline materials may be considered as two-phase materials, composed of distinct interface and crystalline phases. Further, the possibilities for reacting, coating, and mixing various types of nano materials create the potential for fabricating new composites with nanometer-sized phases and novel properties. For a review on nano-phase materials please refer to R. P. Andres, et al. xe2x80x9cResearch Opportunities on Clusters and Cluster-Assembled Materials,xe2x80x9d in Journal of Materials Research, Vol. 4, 1989, pp.704-736 and A. N. Goldstein, xe2x80x9cHandbook of Nanophase Materials,xe2x80x9d Marcel Dekker, Inc., N.Y., 1997.
Ultra-fine metal particles, despite their usefulness, are sometimes difficult to work with due to their high surface-to-volume ratio which makes them more susceptible to environmental degradation (e.g., oxidation and corrosion) than their bulk counterparts. Some ultra-fine metal powders tend to be pyrophoric and present danger in their manufacture, transportation, handling, and storage.
The techniques for the generation of nanometer-sized particles may be divided into three broad categories: vacuum, gas-phase, and condensed-phase synthesis. Vacuum synthesis techniques include sputtering, laser ablation, and liquid-metal ion sources. Gas-phase synthesis includes inert gas condensation, oven sources (for direct evaporation into a gas to produce an aerosol or smoke of clusters), laser-induced vaporization, laser pyrolysis, and flame hydrolysis. Condensed-phase synthesis includes reduction of metal ions in an acidic aqueous solution, liquid phase precipitation of semiconductor clusters, and decomposition-precipitation of ionic materials for ceramic clusters. Other methods include high-energy milling, mix-alloy processing, chemical vapor deposition (CVD), and sol-gel techniques. All of these techniques have one or more of the following problems or shortcomings:
(1) Most of these prior-art techniques suffer from a severe drawback: extremely low production rates. It is not unusual to find a production rate of several grams a day. Vacuum sputtering, for instance, only produces small amounts of particles at a time. Laser ablation and laser-assisted chemical vapor deposition techniques are well-known to be excessively slow processes. The high-energy ball milling method, known to be a xe2x80x9cquantityxe2x80x9d process, is capable of producing only several kilograms of nano-scaled powders in approximately 100 hours. These low production rates, resulting in high product costs, have severely limited the utility value of nano-phase materials. There is, therefore, a clear need for a faster, more cost-effective method for preparing essentially nanometer-sized powder materials.
(2) Condensed-phase synthesis such as direct reaction of metallic silicon with nitrogen to produce silicon nitride powder requires pre-production of metallic silicon of high purity in finely powdered form. This reaction tends to produce a silicon nitride powder product which is constituted of a broad particle size distribution. Furthermore, this particular reaction does not yield a product powder finer than 100 nm (nanometers) except with great difficulty. Due to the limited availability of pure metallic silicon in finely powdered form, the use of an impure metallic powder necessarily leads to an impure ceramic product. These shortcomings are true of essentially all metallic elements, not just silicon.
(3) Most of the prior-art processes require heavy and/or expensive equipment (e.g., a high power laser source or a plasma generator), resulting in high production costs. In the precipitation of ultra fine particles from the vapor phase, when using thermal plasmas or laser beams as energy sources, the particle sizes and size distribution cannot be precisely controlled.
(4) The conventional mechanical attrition and grinding processes have the disadvantages that powders can only be produced up to a certain fineness and with relatively broad particle-size distribution. As a matter of fact, with the currently familiar large-scale process for manufacturing powders it is rarely possible, or only possible with considerable difficulty, to produce powders having average particle sizes of less than 0.5 xcexcm (microns).
(5) Most of these prior-art processes do not allow for production of environmentally stable metal particles.
A relatively effective technique for producing fine metal particles is atomization. Atomization involves the breakup of a liquid into small droplets, usually in a high-speed jet. The preparation of high-quality powders, including aluminum, copper alloys, nickel alloys, cobalt alloys, zinc alloys and the like has been achieved by using the atomization technology. The breakup of a liquid stream by the impingement of high-pressure jets of water or gas is referred to as water or gas atomization, respectively. Other commonly used atomization techniques include centrifugal atomization, vacuum atomization, and ultrasonic atomization. By judiciously varying the parameters of the atomization process, the particle size, particle size distribution, particle shape, chemical composition and micro-structure of the particles can be varied to meet the requirements of a specific application.
The major components of a typical atomization system include a melting chamber (including a crucible, a heating device, and a melt-guiding pipe) in a vacuum or protective gas atmosphere, an atomizing nozzle and chamber, and powder-drying (for water atomization) or cooling equipment. The metal melt can be poured into first end of a guiding pipe having a second end with a discharging nozzle. The nozzle, normally located at the base of the pipe, controls the shape and size of the metal melt stream and directs it into an atomizing chamber in which the metal stream (normally a continuous stream) is disintegrated into fine droplets by the high-speed atomizing medium, either gas or water. Liquid droplets cool and solidify as they settle down to the bottom of the atomizing chamber. A subsequent collector system may be used to facilitate the separation (from the waste gas) and collection of powder particles.
Environmentally stable metal alloy particles have been produced by using gas atomization of a metal melt; e.g., U.S. Pat. No. 5,073,409 (Dec. 17, 1991), U.S. Pat. No. 5,372,629 (Dec. 13, 1994), U.S. Pat. No. 5,589,199 (Dec. 31, 1996), and U.S. Pat. No. 5,811,187 (Sep. 22, 1998), all to Anderson, et al. The method proposed by Anderson, et al. involves atomizing the melt to form droplets and reacting the droplets downstream of the atomizing location with a reactive gas. The sizes of the powders produced by Anderson""s single-stage atomization method tend to be micron-, rather than nanometer-scaled.
The reason behind this drawback can be understood as follows: When a stream of metal melt is broken up into small droplets, the total surface energy of the melt increases. This is due to the fact that the creation of a droplet necessarily generates a new surface and every surface has an intrinsic surface tension. When droplets are broken down into even smaller droplets, the total surface area of the droplets is further increased, given the same volume of material. This implies that a greater amount of energy must be consumed in creating this greater amount of surface area. Where does this energy come from? An atomizer works by transferring a portion of the kinetic energy of a high-speed atomizing medium to the fine liquid droplets. Because of the recognition that the kinetic energy (K.E.) of a medium with a mass m and velocity xcexd is given by K.E.=xc2xd mxcexd2, prior-art atomization technologies have emphasized the importance of raising the velocity of the atomizing medium when exiting an atomizing nozzle. In an industrial-scale atomizer jet nozzle, the maximum velocity of a jetting medium is limited, typically from 60 feet/sec to supersonic velocities. The latter high speeds can only be achieved with great difficulties, by using heavy and expensive specialty equipment. In most of the cases, low atomizing medium speeds led to excessively large powder particles (micron sizes or larger).
How do we overcome this drawback? The effect of temperature on the surface tension of metal melt droplets has been largely overlooked in the prior-art atomization technologies. Hitherto, the metal melts to be atomized for the purpose of producing fine metal powders have been typically super-heated to a temperature higher than the corresponding melting point by an amount of 70 to 300xc2x0 C. (135 to 572xc2x0 F.); e.g., as indicated in U.S. Pat. No. 5,863,618 (Jan. 26, 1999) issued to Jarosinsky, et al.. It is important to recognize that the higher the metal melt temperature is the lower its surface tension. A metal melt at a temperature near its vaporization point has a critically small surface tension (almost zero). This implies that a highly super-heated metal melt can be readily atomized to nanometer-scaled droplets without requiring a high atomizing medium speed. Prior-art technologies have not taken advantage of this important feature. In actuality, it is extremely difficult, if not impossible, for prior-art atomization techniques to make use of this feature for several reasons. Firstly, the vaporization temperature of a metal is typically higher than its melting temperature by one to three thousands of degrees K. The metal melt has to be super-heated to an extremely high temperature to reach a state of very low surface tension. In a traditional atomization apparatus, it is difficult to heat a bulk quantity of metal in a crucible above a temperature higher than 3,500xc2x0 C. (3,773xc2x0 K.), even with induction heating. Second, in a traditional atomization apparatus, the metal melt must be maintained at such a high temperature for an extended period of time prior to being introduced into an atomizer chamber. This requirement presents a great challenge as far as protection of the metal melt against oxidation (prior to atomization) is concerned since oxidation rate is extremely high at such an elevated temperature. Third, such a high-temperature metal melt would have a great tendency to create severe erosion to the wall of the melt-guiding pipe through which the melt is introduced into an atomizer chamber. Very few materials, if any, will be able to withstand a temperature higher than 5,500xc2x0 C., for example, to be selected as a guiding pipe for a refractory metal melt such as tungsten and tantalum. Fourth, the operations of pouring and replenishing a crucible with metal melt implies that the traditional atomization can only be a batch process, not a continuous process and, hence, with a limited production rate. In contrast, provided in the present invention is an effective two-stage method of producing highly super-heated metal melt droplets (first stage) and further breaking up these droplets into mostly nanometer-scaled particles while simultaneously inflicting the formation of a thin protective layer on these nanometer-scaled particles (second stage) by introducing an atomizing medium. The atomizing medium is composed of at least a reactive gas and an inert gas with a small reactive-to-inert gas ratio. The reactive gas selectively reacts with a reactive alloying element contained in the metal melt.
Accordingly, one object of the present invention is to provide an improved method for producing ultra fine and environmentally stable metal powder materials at the nanometer-scale. The method makes use of the concepts of a more effective particle kinetic energy transfer and reduced surface tension.
Another object of the present invention is to provide a method for producing a wide range of ultra fine metal powder materials at a high production rate.
A preferred embodiment of the present invention entails a two-stage method for producing nanometer-scaled, environmentally stable metal powders. In the first stage, the process begins with super-heating a molten metal (containing a reactive alloying element) to an ultra-high temperature (e.g., higher than its melting point by 1,000 to 3,000xc2x0 K.) and breaking up the melt into fine liquid droplets. This stream of highly super-heated metal melt droplets is then introduced into a second-stage atomizer chamber where these fine droplets are further broken up (atomized) into nanometer-sized droplets by a more effective atomizer. This second-stage atomizer preferably comprises a vortex jet nozzle that receives a pressurized atomizing fluid medium from fluid medium suppliers (e.g., compressed gas cylinders) and discharges the fluid medium through an outlet (an orifice or a multiplicity of orifices) into the atomizer chamber. This outlet is preferably annular in shape and engulfing the perimeter of the stream of super-heated metal melt droplets, i.e., coaxial with the droplet stream. When the stream of metal melt droplets are supplied into the atomizer chamber, the pressurized fluid medium, also referred to as the atomizing medium, is introduced through the jet nozzle to impinge upon the stream of super-heated metal droplets to further atomize the melt droplets into nanometer sizes. The atomizing medium is composed of at least a reactive gas and an inert gas at a predetermined proportion; the reactive-to-inert gas ratio being small. The reactive gas is allowed to selectively react with the reactive alloying element in the metal melt to form a thin protective layer on the surface of these nanometer-sized droplets. These nanometer-sized droplets are then rapidly cooled and collected as solid powders.
The first-stage heating and atomizing means preferably includes a thermal spray device selected from the group consisting of an arc spray device, a plasma spray device, a gas combustion spray device, an induction heating spray device, a laser-assisted spray device, and combinations thereof. Further preferably, the thermal spray device is a twin-wire arc spray device. The twin-wire arc spray process, originally designed for the purpose of spray coating, can be adapted for providing a continuous stream of super-heated metal melt droplets. This is a low-cost process that is capable of readily heating up the metal wire to a temperature as high as 6,000xc2x0 C. A pressurized carrier gas is introduced to break up the metal melt into fine droplets, typically 5-200 xcexcm in diameter. In an electric arc, the metal is rapidly heated to an ultra-high temperature and is broken up essentially instantaneously. The duration of time for the metal to stay at a super-heated temperature prior to being atomized at the second-stage is very short, thereby effectively alleviating the potential problem of undesired oxidation. Since the wires can be continuously fed into the arc-forming zone, the arc spray is a continuous process, which means a high production rate of ultra-fine powders.
During the first-stage, the super-heated metal liquid droplets are preferably heated to a temperature at least two times the melting point of the metal when expressed in terms of degrees Kelvin. Further preferably, the super-heated metal liquid droplets are at a temperature that lies between two times and 3.5 times the melting point of the metal when expressed in terms of degrees Kelvin. This could mean a temperature as high as 6,000xc2x0 C. to ensure that the metal melt has a very small surface tension. This is readily achieved by using a thermal spray nozzle in the practice of the present invention. In contrast, in a prior-art atomizer system, it is difficult to use a furnace or induction generator to heat a crucible of metal to a temperature higher than 2,500xc2x0 C.
The presently invented process is applicable to essentially all metallic materials,. When high service temperatures are not required, the metal may be selected from the low melting point group consisting of bismuth, cadmium, cesium, gallium, indium, lead, lithium, rubidium, selenium, tellurium, tin, and zinc. When a high service temperature is required, a metallic element may be selected from the high-melting refractory group consisting of tungsten, molybdenum, tantalum, hafnium and niobium. Other metals with intermediate melting points such as copper, zinc, aluminum, iron, nickel and cobalt may also be selected.
In the second-stage atomizing device, the atomizing fluid medium may include water to achieve water atomization. Gas atomization is preferred, however. Preferably, the jet nozzle in a gas atomization device is a vortex jet nozzle for a more efficient atomization action. Preferably the atomizing fluid medium includes an inert gas selected from the group consisting of argon and helium and a reactive gas selected from the group consisting of hydrogen, oxygen, carbon, nitrogen, chlorine, fluorine, boron, sulfur, and combinations thereof. Argon and helium are noble gases and can be used as a purely atomizing gas (without involving any chemical reaction) to produce fine metal powders. The other gases may be used to react with the reactive alloying element of a metal melt to form a protective layer of hydride, oxide, carbide, nitride, chloride, fluoride, boride, and sulfide, respectively. The reactive element may be the metal element itself (e.g., in the case of pure aluminum melt).
Specifically, if the atomizing fluid medium contains a reactive gas (e.g., oxygen), this reactive gas will also rapidly react with the reactive alloying element in the super-heated metal melt (in the form of fine droplets) to form thin oxide layer. If the atomizing fluid contains a mixture of two or more reactive gases (e.g., oxygen and nitrogen), the resulting protective layer could contain a mixture of oxide and nitride. If the metal melt is a metal alloy or mixture (e.g., containing both indium and tin elements) and the reactive gas is oxygen, the resulting product could contain ultra-fine indium and tin particles with their respective indium-oxide and tin-oxide protective layers.
The method preferably further includes a step of collecting the cooled powder particles in a powder collector system composed of at least one cyclone and a device for separating exhaust gases from solid particles.
Advantages of the present invention may be summarized as follows:
1. A wide variety of nano-scaled, environmentally stable metallic particles can be readily produced. The starting metal materials can be selected from any element in the periodic table that is considered to be metallic. The corresponding partner gas reactants may be selected from the group of hydrogen, oxygen, carbon, nitrogen, chlorine, fluorine, boron, and sulfur to form, as a protective layer, metal hydrides, oxides, carbides, nitrides, chlorides, fluorides, borides, and sulfides, respectively. No known prior-art technique is so versatile in terms of readily producing so many different types of nano-scaled metallic powders.
2. The presently invented process makes use of the concept that a metal melt, when super- heated to an ultra-high temperature (e.g., reaching 2 to 3.5 times its melting temperature in degrees K.) has a negligibly small surface tension so that a melt stream can be easily broken up into nano-scaled clusters or droplets without involving expensive or heavy atomizing nozzle equipment that is required to create an ultra-high medium speed. Prior-art atomization apparatus featuring a crucible for pouring metal melt into a melt-guiding pipe are not capable of reaching such a high super-heat temperature and/or making use of this low surface tension feature due to the four major reasons discussed earlier in the BACKGROUND section.
3. The metal melt can be an alloy of two or more elements which are uniformly dispersed. When broken up into nano-sized clusters, these elements remain uniformly dispersed and are capable of reacting with selected reactant species to form uniformly mixed stable powder particles. No post-fabrication mixing is necessary.
4. The near-zero surface tension also makes it possible to generate metal clusters of relatively uniform sizes, resulting in the formation of stable powders of a narrow particle size distribution.
5. The method involves integration of super-heating, atomizing, and reacting steps into one single operation. This feature, in conjunction with the readily achieved super-heat conditions, makes the method fast and effective and now makes it possible to mass produce nano-sized, environmentally stable powders cost-effectively.
6. The apparatus needed to carry out the invented process is simple and easy to operate. It does not require the utilization of heavy and expensive equipment. The over-all product costs are very low.